Field of the Invention
Cholesteric liquid crystals find a wide variety of applications based on their ability to scatter light selectively through a specific temperature range. The liquid crystals normally go from a liquid transparent phase through a mesophase, where light is selectively scattered and color changes occur with increasing temperature, and then back to a transparent liquid phase.
For liquid crystal compositions which have mesophases substantially above ambient temperatures, the compositions are frequently crystalline at ambient temperatures. Liquid crystal compositions with elevated mesophase ranges, which are crystalline at ambient temperatures have a number of undesirable properties.
When a film of a liquid crystal composition crystallizes, the film appears mottled and pockmarked rather than having a smooth uniform texture. In addition, it is found that the temperature at which the liquid crystal composition begins the mesophase with increasing temperature is different from the temperature at which the liquid crystal composition leaves the mesophase on decreasing the temperature. That is, one obtains a different temperature range for the mesophase depending upon whether one is heating the sample from below the mesophase or cooling the sample from above the mesophase temperature range.
In many elevated temperature applications for cholesteric liquid crystals, one is concerned about the appearance of the film. Also, in order to have reproducibility, it is desirable that the color change for the cholesteric liquid crystal composition relate to the same temperature range, irrespective of whether one is cooling or heating the sample. It is therefore desirable to have cholesteric liquid crystals which provide mesophase ranges substantially above ambient temperatures, but do not crystallize at ambient temperatures when standing over long periods of time.